1. Field of the Invention
The present invention relates to wireless communications. More specifically, the invention relates to canceling transmitter signal energy in a receiver bandwidth.
2. Description of the Related Art
There is a growing need for low cost, high performance radio transceivers that can operate in a full duplex mode in support of such applications as code division multiple access (CDMA), global system for mobile communications (GSM) and time division multiple access (TDMA) remote units and base stations.
Currently, a standard transceiver comprises a transmitter, a receiver and a duplexer. One problem with standard transceivers is some of the transmit signal leaks into the receiver, which corrupts the received signal processed by the receiver.
In some transceivers, the duplexer functions to reduce the transmit signal received by the receiver through the use of one or more circulators, receive filters and/or transmit filters. For example, a circulator within the duplexer directs the transmit signal toward an antenna radiator, while a receive filter rejects the residual transmit energy directed toward the receiver that falls outside of the receive filter bandwidth. Alternatively, some duplexers do not use a circulator.
Imperfect duplexers create two problems. First, the high-level transmit signal from the transmitter drives the receiver into a non-linear operating region. This problem can be solved by increasing the rejection characteristics of the receive filter to more effectively attenuate the main spectral lobe of the transmitter signal. Alternatively, this problem may be mitigated through the use of a high dynamic range, direct conversion, digital receiver that can implement the requisite filter characteristics with lossless digital filtering.
A second problem is noise from the transmitter leaks into the receiver and raises its noise figure. The second problem can be mitigated by further filtering the transmitter output and by reducing the spectral sidelobe energy through signal design and linearization of a transmit amplifier. These changes, however, are undesirable from the standpoint of transmitter efficiency.
Canceling an undesired signal with an adaptive canceller was introduced by B. Widrow et al. in Adaptive Noise Canceling Principles and Applications, Proc. IEEE, Vol. 63, pp. 1269-1716, December 1975, which is hereby incorporated herein by reference. Specific applications for adaptive noise canceling and echo cancellation in data transmission over telephone channels are given by J.G. Proakis et al. in Advanced Digital Signal Processing, Macmillan Publishing Co., New York, 1992, pp. 322-327, 331-332.
The present invention relates to a method and apparatus for adaptive digital cancellation of a transmit signal spectrum in a receiver bandwidth. The apparatus relates to a digital coherent spectral canceller that attenuates the spectral components from the transmitter that fall within the bandwidth of the receiver. The digital adaptive coherent spectral canceller digitizes both a corrupted receiver signal and a reference transmit signal and then digitally implements an adaptive coherent spectral canceller adaptation module.
One embodiment of the invention is implemented with a direct conversion digital receiver, which achieves an image-free, high dynamic range without the use of automatic gain control. Automatic gain control used to extend the dynamic range of a receiver may be undesirable for spectrally crowded applications such as cellular communications because the automatic gain control may make the receiver sensitivity dependent upon signals and interference that are outside the signal channel. For example, it is possible for a strong signal in an adjacent channel to capture the receiver front end and desensitize the receiver such that a weak signal in the channel of interest is undetectable. This is particularly harmful in a base station receiver where the receiver receives incoming signals from multiple remote units. Furthermore, the use of automatic gain control will likely require the digital coherent canceller to track the gain changes which will introduce errors and noise.
One embodiment of the invention uses an adaptive transversal filter in an all digital implementation to compensate for the amplitude and phase differences between the transmitter-to-receiver leakage path and the transmit signal path utilized as a reference over the receiver bandwidth or bandwidth of interest.
An advantage of the invention is reduced complexity and cost, which is achieved through: (1) reduced performance requirements of the duplexer, (2) reduction in the requirement for transmit signal filtering, and (3) reduction in the requirements for high linearity or linearization of the transmit amplifier.
One aspect of the invention relates to an adaptive, coherent, digital canceller system. The canceller system is configured to attenuate a signal spectrum from a transmitter which falls within a bandwidth of a receiver. The canceller system comprises a reference bandpass filter, a reference direct converter, a cross correlation measurer, an adaptation coherent spectral canceller algorithm module executed, for example, on a microcontroller, an adaptive digital transversal filter, and a combiner.
The reference direct converter is adapted to output a digitized transmit signal reference of a spectral energy of the transmitter within the bandwidth of the receiver. The adaptive digital transversal filter is adapted to align an amplitude and phase of a digitized transmit signal reference in a reference path with a transmit signal in a leakage receiver path. The adaptive digital transversal filter outputs a compensated or equalized digitized transmit signal reference. The combiner is adapted to coherently subtract the compensated, digitized transmit signal reference from a corrupted, digitized receiver signal to form a residue, having transmitter spectral signal power within the bandwidth of a receiver is suppressed.
Another aspect of the invention relates to a method of attenuating a transmit signal spectrum in a bandwidth of a receiver. The method comprises digitizing a received signal which is corrupted by components of a transmit signal, creating a digitized reference transmit signal of the transmit signal within the bandwidth of the receiver, aligning the digitized reference transmit signal in amplitude, phase and time delay with the digitized received signal, subtracting the digitized reference transmit signal from the digitized received signal to form a residue, and suppressing a transmitter spectral signal,power of the residue within the bandwidth of the receiver.